1. Field of the Invention
The invention relates to a gas concentration detecting device that is capable of acquiring the accurate concentration of the sulfur oxide (SOx) contained in exhaust gas from an internal combustion engine.
2. Description of Related Art
An air-fuel ratio sensor (A/F sensor) that acquires the air-fuel ratio (A/F) of an air-fuel mixture in a combustion chamber based on the concentration of the oxygen (O2) contained in exhaust gas so as to control an internal combustion engine is in wide use. A limiting current-type gas sensor is an example of this type of air-fuel ratio sensor.
The limiting current-type gas sensor used as the air-fuel ratio sensor described above is provided with a pumping cell that is an electrochemical cell which includes a solid electrolyte body having oxide ion conductivity and a pair of electrodes fixed to surfaces of the solid electrolyte body. One of the pair of electrodes is exposed to the exhaust gas from the internal combustion engine as test gas that is introduced via a diffusion resistance unit and the other one of the pair of electrodes is exposed to the atmosphere. When a voltage equal to or higher than a voltage at which the decomposition of oxygen is initiated (decomposition initiation voltage) is applied between the pair of electrodes with one of the pair of electrodes being a cathode and the other one of the pair of electrodes being an anode, the oxygen contained in the test gas becomes an oxide ion (O2−) through reductive decomposition. This oxide ion is conducted to the anode via the solid electrolyte body, becomes oxygen, and is discharged into the atmosphere. This oxygen movement based on the conduction of the oxide ion via the solid electrolyte body from the cathode side to the anode side is referred to as an “oxygen pumping action”.
The conduction of the oxide ion resulting from the oxygen pumping action causes a current flow between the pair of electrodes. This current that flows between the pair of electrodes is referred to as an “electrode current”. This electrode current tends to become stronger as the voltage applied between the pair of electrodes (hereinafter, simply referred to as an “applied voltage” in some cases) increases. However, the flow rate of the test gas reaching the electrode (cathode) is limited by the diffusion resistance unit, and thus the rate of consumption of the oxygen resulting from the oxygen pumping action exceeds the rate of supply of the oxygen to the cathode soon. In other words, the reductive decomposition reaction of the oxygen in the cathode reaches a diffusion rate-controlled state.
In the diffusion rate-controlled state, the electrode current does not increase but remains substantially constant despite a rise in the applied voltage. The characteristics are referred to as “limiting current characteristics” and the range of the applied voltage in which the limiting current characteristics are expressed (observed) is referred to as a “limiting current region”. The electrode current in the limiting current region is referred to as a “limiting current” and the magnitude of the limiting current (limiting current value) is correlated with the rate of the supply of the oxygen to the cathode. Since the flow rate of the test gas reaching the cathode is maintained to be constant by the diffusion resistance unit as described above, the rate of the supply of the oxygen to the cathode is correlated with the concentration of the oxygen contained in the test gas.
Accordingly, in the limiting current-type gas sensor used as the air-fuel ratio sensor, the electrode current (limiting current) pertaining to a case where the applied voltage is set to the “predetermined voltage in the limiting current region” is correlated with the concentration of the oxygen contained in the test gas. By using the limiting current characteristics of the oxygen described above, the air-fuel ratio sensor can detect the concentration of the oxygen contained in the test gas and acquire the air-fuel ratio of the air-fuel mixture in the combustion chamber based thereon.
The limiting current characteristics described above are not characteristics limited to oxygen gas. Specifically, the limiting current characteristics can be expressed based on an appropriate selection of the applied voltage and a cathode configuration in some of gases containing oxygen atoms in molecules (hereinafter, referred to as “oxygen-containing gases” in some cases). Examples of the oxygen-containing gases include sulfur oxide (SOx), water (H2O), and carbon dioxide (CO2).
A fuel for the internal combustion engine (such as light oil and gasoline) contains a small amount of a sulfur (S) component. Especially, a fuel that is also referred to as a poor fuel may have a relatively high sulfur component content. When the sulfur component content (hereinafter, simply referred to as a “sulfur content” in some cases) of a fuel is high, the likelihood of problems increases such as the degradation and/or malfunctioning of members constituting the internal combustion engine, poisoning of an exhaust gas purification catalyst, and generation of white smoke in the exhaust gas. Accordingly, it is desirable that the sulfur component content of the fuel is acquired so that the acquired sulfur content is, for example, reflected in controlling the internal combustion engine, used in issuing a warning relating to the malfunctioning of the internal combustion engine, or utilized in improving the on-board diagnosis (OBD) of the exhaust gas purification catalyst.
When the fuel for the internal combustion engine contains the sulfur component, sulfur oxide is contained in the exhaust gas that is discharged from the combustion chamber. In addition, the concentration of the sulfur oxide contained in the exhaust gas (hereinafter, simply referred to as a “SOx concentration” in some cases) increases as the content of the sulfur component in the fuel (sulfur content) increases. Accordingly, it is considered that an accurate sulfur content can be acquired based on the acquired SOx concentration when an accurate SOx concentration in the exhaust gas can be acquired.
In this technical field, attempts have been made to acquire the concentration of sulfur oxide contained in exhaust gas from an internal combustion engine by using the limiting current-type gas sensor that uses the oxygen pumping action described above. Specifically, a limiting current-type gas sensor (two cell- and limiting current-type gas sensor) is in use that is provided with two pumping cells which are arranged in series with cathodes facing each other in an internal space into which the exhaust gas from the internal combustion engine is introduced as test gas via the diffusion resistance unit.
In this sensor, the oxygen contained in the test gas is removed by the oxygen pumping action of the upstream-side pumping cell when a relatively low voltage is applied between the electrodes of the upstream-side pumping cells. In addition, the sulfur oxide contained in the test gas is subjected to reductive decomposition in the cathode by the downstream-side pumping cell when a relatively high voltage is applied between the electrodes of the downstream-side pumping cells, and the oxide ion that is generated as a result is conducted to the anode. The concentration of the sulfur oxide contained in the test gas is acquired based on the change in the electrode current value attributable to the oxygen pumping action (for example, refer to Japanese Patent Application Publication No. 11-190721).